The present invention relates to a weight measuring apparatus comprising a weight transducer of vibration type such as a vibrating string transducer and a tuning fork transducer.
A weight transducer comprising a vibrating string or a tuning fork has several advantages that the construction is simple and it does not require an analog-to-digital converter, because it produces a measured result directly as a digital value, i.e. the number of vibration waves. However, such a weight transducer of vibration type is very sensitive to disturbing vibration and impact applied from the external and therefore, a measured value displayed on a digital display device is liable to fluctuate and is not easily read out stably. Thus the weighing apparatus of vibration type has not been used widely.
FIG. 1 is a schematic view showing an example of the known vibration type weighing apparatus comprising a weight transducer of vibration type. The weight transducer comprising a tuning fork 1 which has a pair of vibrating strips 1a and 1b arranged symmetrically with respect to a central axis in parallel with each other. Both ends of the vibrating strips 1a and 1b are connected to each other by means of U-shaped coupling members 2a and 2b. On the both surfaces of leg portions of the lower coupling member 2b are fixed first and second piezo-electric elements 3a and 3b which are connected to an oscillator and amplifier 4. The first piezo-electric element 3a is used as a vibration pick-up element and the second piezo-electric element 3b functions to exite the tuning fork 1.
In the known weighing apparatus of the type mentioned above, by suitably selecting gain and frequency characteristics of the oscillator and amplifier 4, the vibrating strips 1a and 1b oscillate at a fundamental frequency in a symmetrical mode as illustrated by broken lines in FIG. 1. Under such condition, when a weight F to be measured is applied to the tuning fork 1 via supporting members 5a and 5b in the axial direction, vibration frequency of the tuning fork 1 is varied corresponding to the magnitude of the weight F. Therefore, by detecting the vibration frequency of the tuning fork 1 by means of a frequency counter 6, it is possible to measure the applied weight F. However, in practice, to the tuning fork 1 might be applied various disturbing vibrations and impacts causing sudden changes in an acceleration force such as forces f.sub.1 , f.sub.2 in the axial direction and forces f.sub.3 , f.sub.4 in directions perpendicular to the axial direction. Then the frequency would be changed abruptly to a great extent and the normal vibrationg condition is disturbed greatly. Therefore, an accurate relation between the weight F to be measured and the frequency might be lost. Under such an abnormal condition, it is no more possible to effect the accurate measurement by known measures such that a filter is provided before the frequency counter 6 or a period for counting the vibration waves is prolonged. Therefore, the reliability of measurement is entirely lost and the displayed value could not be read stably due to the disturbing vibration.